This study is investigating photochemically-induced cell lethality and mutagenesis (6-thioguanine resistance) in normal and xeroderma pigmentosum human diploid cells as a function of wavelength. Selected wavelengths in the far UV, near UV, and visible range are being used. We are investigating the role of DNA damage and repair phenomena in cell lethality and mutagenesis. At each wavelength, DNA strand breaks, pyrimidine dimers, excision resynthesis, and the average size of repair-synthesized DNA regions will be determined. The oxygen dependence of cell lethality, mutagenesis, and DNA damage and repair is also being measured as a function of wavelength. This will allow determination of the contribution of direct and photodynamic damage mechanisms to these radiation effects. The overall objectives of this study are to measure lethality and mutagenesis induced by radiations between 254 and 550 nm, and to investigate the basic mechanisms involved.